Different techniques are known for three dimensional imaging.
It is known to carry out three dimensional particle imaging with a single camera. This is also called quantative volume imaging. One technique, described by Willert and Gharib uses a special defocusing mask relative to the camera lens. This mask is used to generate multiple images from each scattering site on the item to be imaged. This site can include particles, bubbles or any other optically-identifiable image feature. The images are then focused onto an image sensor e.g. a charge coupled device, CCD. This system allows accurately, three dimensionally determining the position and size of the scattering centers.
Another technique is called aperture coded imaging. This technique uses off-axis apertures to measure the depth and location of a scattering site. The shifts in the images caused by these off-axis apertures are monitored, to determine the three-dimensional position of the site or sites.
There are often tradeoffs in aperture coding systems.
FIG. 1A shows a large aperture or small f stop is used. This obtains more light from the scene, but leads to a small depth of field. The small depth of field can lead to blurring of the image. A smaller F stop increases the depth of field as shown in FIG. 1B. Less image blurring would therefore be expected. However, less light is obtained.
FIG. 1C shows shifting the apertures off the axis. This results in proportional shifts on the image plane for defocused objects.
The FIG. 1C system recovers, the three dimensional spatial data by measuring the separation between images related to off-axis apertures b, to recover the “z” component of the images. The location of the similar image set is used find the in-plane components x and y.
Systems have been developed and patented to measure two-component velocities within a plane. Examples of such systems include U.S. Pat. Nos. 5,581,383, 5,850,485, 6,108,458, 4,988,191, 5,110,204, 5,333,044, 4,729,109, 4,919,536, 5,491,642. However, there is a need for accurately measuring three-component velocities within a three-dimensional volume. Prior art has produced velocimetry inventions, which produce three-component velocities within a two-dimensional plane. These methods are typically referred to as stereo imaging velocimetry, or stereoscopic velocimetry. Many such techniques and methods have been published, i.e. Eklins et al. “Evaluation of Stereoscopic Trace Particle Records of Turbulent flow Fields” Review of Scientific Instruments, vol. 48, No. 7, 738-746 (1977); Adamczyk & Ramai “Reconstruction of a 3-Dimensional Flow Field” Experiments in Fluids, 6, 380-386 (1988); Guezennec, et al. “Algorithms for Fully Automated Three Dimensional Tracking Velocimetry”, Experiments in Fluids, 4 (1993).
Several stereoscopic systems have also been patented. Raffel et al., under two patents, U.S. Pat. Nos. 5,440,144 and 5,610,703 have described PIV (Particle Image Velocimetry) systems for measuring three-component velocities within a two-dimensional plane. U.S. Pat. No. 5,440,144 describes an apparatus using 2 cameras, while U.S. Pat. No. 5,610,703 describes an apparatus and method using only one camera to obtain the three-component velocity data. U.S. Pat. No. 5,905,568 describes a stereo imaging velocimetry apparatus and method, using off-the-shelf hardware, that provides three-dimensional flow analysis for optically transparent fluid seeded with tracer particles.
Most recently, a velocimetry system that measures three-component velocities within a three-dimensional volume has been patented under U.S. Pat. No. 5,548,419. This system is based upon recording the flow on a single recording plate by using double exposure, double-reference-beam, and off-axis holography. This system captures one velocity field in time, thereby preventing acquisition through time, and analysis of time evolving flows.
There therefore still exists a need for a system and method by which accurate three-component velocities can be obtain within a three-dimensional volume using state-of-the-art analysis for any optically transparent fluids seeded with tracer particles.
Three-Dimensional Profilometry is another technique, often used for measuring the three-dimensional coordinate information of objects: for applications in speeding up product development, manufacturing quality control, reverse engineering, dynamical analysis of stresses and strains, vibration measurements, automatic on-line inspection, etc. Furthermore, new fields of application, such as computer animation for the movies and game markets, virtual reality, crowd or traffic monitoring, biodynamics, etc, demand accurate three-dimensional measurements. Various techniques exist and some are now at the point of being commercialized. The following patents describe various types of three-dimensional imaging systems:
U.S. Pat. No. 3,589,815 to Hosterman, Jun. 29, 1971;
U.S. Pat. No. 3,625,618 to Bickel, Dec. 7, 1971;
U.S. Pat. No. 4,247,177 to Marks et al, Jan. 27, 1981;
U.S. Pat. No. 4,299,491 to Thornton et al, Nov. 10, 1981;
U.S. Pat. No. 4,375,921 to Morander, Mar. 8, 1983;
U.S. Pat. No. 4,473,750 to Isoda et al, Sep. 25, 1984;
U.S. Pat. No. 4,494,874 to DiMatteo et al, Jan. 22, 1985;
U.S. Pat. No. 4,532,723 to Kellie et al, Aug. 6, 1985;
U.S. Pat. No. 4,594,001 to DiMatteo et al, Jun. 10, 1986;
U.S. Pat. No. 4,764,016 to Johansson, Aug. 16, 1988;
U.S. Pat. No. 4,935,635 to O'Harra, Jun. 19, 1990;
U.S. Pat. No. 4,979,815 to Tsikos, Dec. 25, 1990;
U.S. Pat. No. 4,983,043 to Harding, Jan. 8, 1991;
U.S. Pat. No. 5,189,493 to Harding, Feb. 23, 1993;
U.S. Pat. No. 5,367,378 to Boehnlein et al, Nov. 22, 1994;
U.S. Pat. No. 5,500,737 to Donaldson et al, Mar. 19, 1996;
U.S. Pat. No. 5,568,263 to Hanna, Oct. 22, 1996;
U.S. Pat. No. 5,646,733 to Bieman, Jul. 8, 1997;
U.S. Pat. No. 5,661,667 to Bordignon et al, Aug. 26, 1997; and
U.S. Pat. No. 5,675,407 to Geng, Oct. 7, 1997.
U.S. Pat. No. 6,252,623 to Lu, Jun. 26, 2001.
If contact methods are still a standard for a range of industrial applications, they are condemned to disappear: as the present challenge is on non-contact techniques. Also, contact-based systems are not suitable for use with moving and/or deformable objects, which is the major achievement of the present method. In the non-contact category, optical measurement techniques are the most widely used and they are constantly updated, in terms of both of concept and of processing. This progress is, for obvious reasons, parallel to the evolution observed in computer technologies, coupled with the development of high performance digital imaging devices, electro-optical components, lasers and other light sources.
The following briefly describe techniques:
The time-of-flight method is based on the direct measurement of the time of flight of a laser or other light source pulse, e.g. the time between its emission and the reception time of the back reflected light. A typical resolution is about one millimeter. Light-in-flight holography is another variant where the propagating optical wavefront is regenerated for high spatial resolution interrogation: sub-millimeter resolution has been reported at distances of 1 meter. For a surface, such technique would require the scanning of the surface, which of course is incompatible with the measurement of moving objects.
Laser scanning techniques are among the most widely used. They are based on point laser triangulation, achieving accuracy of about 1 part in 10000. Scanning speed and the quality of the surface are the main factors against the measurement accuracy and system performance.
The Moiré method is based on the use of two gratings, one is a reference (i.e. undistorted) grating, and the other one is a master grating. The typical measurement resolution is 1/10 to 1/100 of a fringe in a distance range of 1 to 500 mm.
Interferometric shape measurement is a high accuracy technique capable of 0.1 mm resolution with 100 m range, using double heterodyne interferometry by frequency shift. Accuracies 1/100 to 1/1000 of fringe are common. Variants are under development: shearography, diffraction grating, wavefront reconstruction, wavelength scanning, conoscopic holography.
Moiré and interferometer based systems provide a high measurement accuracy. Both, however, may suffer from an inherent conceptual drawback, which limits depth accuracy and resolution for surfaces presenting strong irregularities. In order to increase the spatial resolution, one must either use shift gratings or use light sources with different wavelengths. Three to four such shifts are necessary to resolve this limitation and obtain the required depth accuracy. This makes these techniques unsuitable for time-dependent object motion. Attempts have been made with three-color gratings to perform the Moiré operation without the need for grating shift. However, such attempts have been unsuccessful in resolving another problem typical to fringe measurement systems: the cross-talk between the color bands. Even though some systems deliberately separate the bands by opaque areas to solve this problem, this is done at the expense of a much lower spatial resolution.
Laser radar 3D imaging, also known as laser speckle pattern sampling, is achieved by utilizing the principle that the optical field in the detection plane corresponds to a 2D slice of the object's 3D Fourier transform. Different slices can be obtained by shifting the laser wavelength. When a reference plane is used, this method is similar to two-wavelength or multi-wavelength speckle interferometry. The measurement range goes from a micrometer to a few meters. Micrometer resolutions are attained in the range of 10 millimeters.
Photogrammetry uses the stereo principle to measure 3D shape and requires the use of bright markers, either in the form of dots on the surface to be measured of by projection of a dot pattern. Multiple cameras are necessary to achieve high accuracy and a calibration procedure needs to be performed to determine the imaging parameters of each of them. Extensive research has been done on this area and accuracies in the order of one part in 100000 are being achieved. Precise and robust calibration procedures are available, making the technique relatively easy to implement.
Laser trackers use an interferometer to measure distances, and two high accuracy angle encoders to determine vertical and horizontal encoders. There exist commercial systems providing accuracies of +/−100 micrometers within a 35-meter radius volume.
Structured light method is a variant of the triangulation techniques. Dots or lines or projected onto the surface and their deformed pattern is recorded and directly decoded. One part over 20000 has been reported.
Focusing techniques that have received a lot of attention because of their use in modern photographic cameras for rapid autofocusing. Names like depth-from-focus and shape-from-focus have been reported. These techniques may have unacceptably low accuracy and the time needed to scan any given volume with sufficient resolution have confined their use to very low requirement applications.
Laser trackers, laser scanning, structured light and time-of-flight methods require a sweeping of the surface by the interrogation light beam. Such a scanning significantly increases the measuring period. It also requires expensive scanning instruments. The Moiré technique requires very high resolution imaging devices to attain acceptable measurement accuracy. Laser speckle pattern sampling and interferometric techniques are difficult and expensive to implement. For large-scale measurements, they require also more time to acquire the image if one wants to take advantage of the wavelength shifting method. Photogrammetry needs a field calibration for every configuration. Furthermore, the highest accuracy is obtained for large angular separations between the cameras, thus increasing the shading problem.
There is thus a widely recognized need for a method and system to rapidly, accurately and easily extract the surface coordinate information of as large as possible number of designated features of the scene under observation, whether these features are stationary, in motion, and deforming. The technique should be versatile enough to cover any range of measurement, and with accuracy comparable to or surpassing that of systems available today. The technique should allow for fast processing speeds. Finally, the technique should be easy to implement for the purpose of low cost manufacturing. As we will describe, the present invention provides a unique alternative since it successfully addresses these shortcomings, inherent partially or totally to the presently know techniques.